As one of image forming devices, there is an inkjet recording device that forms an image by discharging ink from a plurality of nozzles of an inkjet head to a recording medium conveyed on a conveyance path. In the inkjet recording device, if there are variations in ink discharge characteristics among the respective nozzles, density unevenness is generated in an image due to the size or displacement of positions of dots formed on a recording medium such as printing paper.
Therefore, it has been known to measure the ink discharge characteristics for each nozzle and calculate a density correction value for canceling the variations in the ink discharge characteristics among the nozzles, to correct a density gradation value of a pixel corresponding to each nozzle by using the calculated correction value.
The density correction value can be calculated, for example, by reading by a scanner, a test pattern image formed on the recording medium by ink droplets discharged from each nozzle, and digitizing the variations in the ink discharge characteristics among the nozzles based on a reading result.
However, when the test pattern image is to be read by the scanner, noise may be superimposed on an image signal, for example, because dark color fiber mixed into the printing paper or dirt on a glass placing table on which an original document to be read is placed is read together with the test pattern image. Therefore, smoothing processing such as a moving average is applied to an image signal as the reading result by the scanner to remove a noise component in the image signal.
The smoothing processing with respect to the image signal as the reading result of the test pattern image by the scanner is performed similarly even when variations in high frequency components of density generated between adjacent pixels due to a sharp change of the ink discharge characteristics of the nozzle are included in the image signal as the reading result of the test pattern image by the scanner. The variations in high frequency components of density appear, for example, when the ink discharge characteristics of a certain nozzle are largely different from those of the adjacent nozzles on both sides.
If the variations in the high frequency components of density are included in the image signal as the reading result of the test pattern image by the scanner, even if the density gradation values of pixels corresponding to the respective nozzles are corrected by the density correction value calculated based on the image signal as the reading result by the scanner after the smoothing processing, density unevenness in the image due to variations in the ink discharge characteristics of the high frequency components cannot be sufficiently eliminated.
Therefore, according to Patent Literature 1, it has been proposed to extract a high-frequency band from an image signal as a reading result by a scanner and remove noise components from the extracted high-frequency band of the image signal by a filter or the like, instead of applying the smoothing processing to the image signal as the reading result by the scanner.
According to the proposal disclosed in Patent Literature 1, after the high-frequency band of the image signal having the noise components removed is synthesized with other frequency bands of the image signal, a density correction value of the density gradation value of pixels corresponding to respective nozzles is calculated from the synthesized image signal.